High capacitance sheet adhesives

ABSTRACT

The present invention relates to thin adhesive composite films formed from fluoropolymers imbibed with adhesives and containing a filler at least within the infra-structure of the polymer to provide the film with a high dielectric constant. The films of the present invention are particularly suitable for use in a capacitor or in applications requiring high capacitive properties.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 08/311,634, to Korleski et al., filed Sep. 23, 1994, U.S. Pat.No. 5,538,756 which relates to co-pending application Ser. No.08/296,220 to Joseph Korleski, filed on Aug. 25, 1994.

FIELD OF THE INVENTION

The present invention relates to thin adhesive composite films formedfrom filled or unfilled polymeric substrates that are imbibed with anadhesive or filler-adhesive mixture such that the film has highdielectric properties. The filler imparts high dielectric constants tothe adhesive without loss of physical properties. Specifically, thepresent invention provides thin fluoropolymer adhesive composite filmshaving high capacitance. The imbibed film composites of the presentinvention are suitable for use in the fabrication of electronic devices.

BACKGROUND OF THE INVENTION

The electronics industry is constantly challenged with the need toreduce the size and weight of electronic devices while increasing theirperformance. One method of providing the necessary increase in densityis to combine functions of the different components. Printed WiringBoards (PWB) are currently used to connect different devices in order toperform an electronic function. It would be desirable if the PWB hadsome of the functionality of some of those devices, eliminating the needto mount them to the surface. Resistors and capacitors are very simpledevices that are required in almost every circuit. Because of thesimplicity of their functions, they are prime candidates forincorporation into a PWB structure.

The desire to incorporate a capacitor into a PWB is not a new concept.Limitations in materials, however, have prevented this from becomingcommon. There are presently several high dielectric materials which canbe used as an internal capacitor, but they all have limitations in theamount of capacitance that they can supply. The capacitance of the layeris a function of the thickness and dielectric constant. An increase indielectric constant and/or a reduction in thickness will increase thecapacitance of the layer.

Copper clad substrates have been made with an epoxy/glass core as thinas 2 mils. This core material has a dielectric constant of 4.5. Thedielectric constant is material dependent, and the material cannot bemade thinner than 2 mils and remain practical for manufacturing. So thistype of material is limited to a maximum capacitance of around 500picofarads/in².

Copper clad substrates have been made with a resin/ceramic core as thinas 10 mils. This core material can have a dielectric constant as high as10. The dielectric constant is material dependent, and the materialcannot be made thinner than 10 mils and remain practical formanufacturing. So this type of material is limited to a capacitance ofaround 220 picofarads/in².

Copper clad substrates have been made with a teflon/ceramic core as thinas 5 mils. This core material can have a dielectric constant as high as10. The dielectric constant is material dependent, and the materialcannot be made thinner than 5 mils and remain practical formanufacturing. So this type of material is limited to a capacitance ofaround 450 picofarads/in².

Additionally, these types of materials have historically displayed verylow voltage breakdown, and, as such present poor reliability as acapacitor.

The limitation on thickness generally results from the need to use thematerial as a copper clad substrate. Thin materials are prone to damageduring the printed circuit fabrication process. It would be desirable touse the material as an adhesive sheet (referred to as a prepreg or "B"stage) instead of a copper clad substrate (referred to as "C" stage).The metal layers would be provided by other substrates in thefabrication of the PWB. So the dielectric would then be a thin adhesivebond line between two metal plates. This would facilitate fabrication aswell as allowing for thinner layers that would have higher capacitance.

The three copper clads listed above all have limitations in making thinadhesive sheets (prepregs).

Epoxy/glass prepregs use a woven glass reinforcement to providedimensional stability. Currently, there is a limitation in themanufacturing of a woven glass material which ultimately limits thethickness of the prepreg. The glass does not compress during laminationbetween two metal planes, so the thickness is limited to the thicknessof the woven glass. In addition, the dielectric constant is limited bythe materials to a high of about 4.5.

High dielectric constant resin/ceramic prepregs are not currentlyavailable because of difficulty in handling. High levels of ceramic areneeded to raise the dielectric constant, and this level of filler in asemi-cured or "B" staged material is too brittle to be used as a largethin sheet adhesive.

PTFE/ceramic prepregs have limitations in processing as well asproperties. The PTFE requires very high temperatures and pressures thatare not compatible with standard PWB processing. In addition, thematerial has poor dielectric breakdown voltage that changes with time.

Thus a need exists for a prepreg film having a high dielectric constant,at least 4.5, that avoids the drawbacks of the prior art and is anunclad, i.e., non-laminated, uncured material that can be bonded toinner layers of circuitry in a convenient fashion, e.g., a prepreg.

The subject invention, described below satisfies these needs and avoidsthe disadvantages of the prior art.

SUMMARY OF THE INVENTION

The present invention is directed to high dielectric adhesive-fillerfilms that reduce or eliminate the need for capacitors in circuit boardsby incorporating high capacitive layers internal to the PWB as apower/ground pair. The adhesive allows ease of processing by the PWBmanufacturer by allowing its use as a bond-ply or "B" stage, thusallowing for customary multilayer fabrication techniques common to thoseskilled in the art.

One aspect of the present invention is to provide a film having adielectric constant (Dk) such that use of the PWB can be significantlyincreased using the mixed dielectric approach, and offer reliableminiaturization of circuitry.

Another aspect of the subject invention is to provide a uniformlyreinforced, thin adhesive sheet composite that is capable of retaininghigh levels of reinforcement by containing the greatest possiblequantity of filler to maximize the desired property without sacrificingstructural integrity.

Still another aspect of the invention is to provide a PTFE "sheet"adhesive layer that bonds directly to metal conductive layers withoutthe use of fusion temperatures or additional adhesives.

These and other objects of the present invention will become evidentfrom review of the following description when considered in conjunctionwith the accompanying non-limiting drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an expanded or stretched polytetrafluoroethylene film(A) containing nodes (B) and interconnected with fibrils (C) with aparticulate dielectric filler (D) located within the node-and-fibrilstructure and adhesive (E) in the void volume of the film.

FIG. 2 illustrates an expanded or stretched polytetrafluoroethylene filmwhere the particulate dielectric filler and adhesive are located withinthe void volume of the film.

FIG. 3 illustrates an expanded or stretched polytetrafluoroethylene filmwhere a dielectric filler is located within the node-and-fibrilstructure and an adhesive with a filler are located in the void volumeof the film.

DETAILED DESCRIPTION OF THE INVENTION

It has now been discovered that high capacitive, very thin,well-controlled thickness adhesives can be obtained by using filledexpanded porous PTFE without producing team or pinholes in the thinfilms. Unusually low loadings of thermosetting or thermoplastic adhesivecan be achieved by the invention described herein, while still providingexceptional adhesion. An added and unexpected result to the invention isthat even small amounts of resin impart elevated voltage breakdownstrength to the composite.

The high dielectric adhesive films of the present invention arepreferably formed from porous, expanded PTFE that contains a filler. Thefiller is present in the range of 5-80% of the final composition,preferably 7 to 63 volume percent, most preferably 15-55% in the form ofparticulate or fibers; 4-85 volume percent, and preferably 10 to 40volume percent PTFE. The adhesive, high dielectric films of the presentinvention have a thickness of 0.2 mils or greater.

An important aspect of the process invention lies in the use of theunusual feature of polytetrafluoroethylene (PTFE) to expand onstretching to form a porous material of interconnecting channels formedby nodes and fibrils. The stretching of polytetrafluoroethylene (PTFE)to form porous material is well known, and is taught in U.S. Pat. No.3,953,566 to Gore and U.S. Pat. No. 4,482,516 to Bowman, et al., each ofwhich is incorporated herein by reference. The void space in expandedPTFE comprises more than 30%, preferably at least 50% of the volume, andfrequently more than 70% of the volume. Because of the expansion, theparticulate filler particles are drawn apart from one another as thePTFE is expanded. This reduces the opportunity for tears or pinholes toform as the PTFE is compressed and results in a thin, highly filledfilm.

Expanded porous PTFE is used as the matrix material to make the filmbecause of both its porosity and its extra strength due to its expandedform. This high strength is not lost when a filler is incorporated intothe nodes of the node-and-fibril structure. Thin porouspolytetrafluoroethylene films containing fillers in the node-fibrilstructure are prepared in accordance with the teachings of U.S. Pat. No.4,985,296 to Mortimer and U.S. Pat. No. 4,996,097 to Fischer, each ofwhich is incorporated herein by reference. The porouspolytetrafluoroethylene used in the Mortimer and Fischer patents isexpanded polytetrafluoroethylene prepared in accordance with theteachings of U.S. Pat. No. 3,953,566 to Gore.

Using such porous films of filled expanded PTFE as a substrate, a bondply of the present invention is created by imbibing an adhesive into theporous structure of the expanded PTFE substrate. While it might beexpected that higher quantities of adhesive fillers would result inbetter adhesive properties, it has been found in the present inventionthat improved adhesion is achieved by limiting the quantity of adhesiveemployed. Although adhesive can be provided up to 85 volume percent ormore, the present invention preferably comprises a fill of only 50volume percent adhesive or less. Most preferably, the adhesive fillshould comprise about 10-50 volume percentage of the final composite,and ideally 15-40 volume percent.

In accordance with the present invention, films filled with a dielectricfiller are prepared according to the teachings of Mortimer and Fischerdescribed above. The adhesive resin is then imbibed into the voids ofthe expanded polytetrafluoroethylene. The result is an exceptionallythin bond-ply "prepreg" with high dielectric constant andwell-controlled thicknesses. The adhesive containing filler-impregnatedarticle is also very compressible. This is highly desirable to take uplocal variances in thickness that may exist between inner layers withina PWB, such as the lines and spaces of etched circuitry.

The features of the present invention can be described by reference tonon-limiting FIGS. 1 through 3. Thus, in FIG. 1 is illustrated a film(A) comprising of nodes (B) and fibrils (C) where the nodes containdielectric particles (D) and the void volume within film (A) is at leastpartially filled with adhesive (E).

In FIG. 2, the dielectric particulate (D) and adhesive (E) are found inthe void volume of film A. The node-and-fibril structure serves as ascaffolding for the filled adhesive.

FIG. 3 illustrated how dielectric particulate (D1 and D2), notnecessarily the same ones, can be found both in the nodes (B) and in thevoid volume of film (A). To facilitate formation of the thin films ofthe present invention, the particulate size of the fillers shouldaverage 40 microns or less. By "particulate" is meant individualparticles of any aspect ratio and thus includes fibers and powders.

To prepare the filled films of the present invention, particulate filleris mixed into an aqueous dispersion of dispersion-produced PTFE. Thefiller in small particle form is ordinarily less than 40 microns insize, and preferably has an average particulate size between 1 an 15microns. The filler is introduced prior to coagulation in an amount thatwill provide 25 to 85, preferably 40 to 85 volume percent filler in thePTFE in relation to each other after the PTFE is coagulated andexpanded.

The filled films are easily imbibed with resin. In this case, all orpart of the void volume comprising air is replaced with an adhesiveresin. The adhesive itself may be a thermoset or thermoplastic and caninclude polyglycidyl ether, polycyanurate, polyisocyanate, bis-triazineresins, poly (bis-maleimide), norbornene-terminated polyimide,acetylene-terminated polyimide, polybutadiene and functionalizedcopolymers thereof, polysiloxanes, poly sisqualoxane, functionalizedpolyphenylene ether, polyacrylate, novolak polymers and copolymers,fluoropolymers and copolymers, melamine polymers and copolymers,poly(bis phenycyclobutane) and blends thereof. It should be understoodthat the aforementioned adhesives may themselves be blended together orblended with other polymers or additives, so as to impact flameretardancy or enhanced toughness. In the case where only part of thevoid volume of air is replaced with resin, the final composite can becompressed in place to a very thin, void-free composite with excellentadhesion. The ultimate thickness, degree of adhesion, and finalcompositional mixture could not be achieved any other way.

To prepare a filled adhesive film of the present invention, particulatefiller is mixed into an aqueous, solvent solution or molten adhesive toafford a finely dispersed mixture. The filler in small particle form isordinarily less than 40 microns in size, and preferably has an averageparticulate size between 1 and 10 microns. The mean pore size of thenode-and-fibril structure should be large enough to allow for adequatepenetration of the particulate. If the substrate is to be an expandedPTFE substrate, then structures similar to those taught in U.S. Pat. No.4,482,516 to Bowman et al are desirable.

The open films are easily imbibed with the above-mentioned ceramicfilledresin. In this case, all or part of the void volume comprising air isreplaced with the ceramic filled resin. In the case where only part ofthe void volume of air is replaced with resin, the final composite canbe compressed in place to a very thin, void-free composite withexcellent adhesion, superior thickness control, and excellentflexibility and compressibility. Thus, in this manner, one is capable ofmaking exceptionally thin, well-controlled thicknesses of unusuallyhighly loaded adhesives which were otherwise unattainable.

One of the important advantages of the present invention is itsexceptional capacitance performance. Typical composite material of thepresent invention will have a capacitance of at least 700 picofarads/in²and a voltage breakdown of greater than 500 volts/mil.

In the present invention, average particle size and largest particlesize were determined using a Microtrac light scattering particle sizeanalyzer Model No. FRA (Microtrac Division of Leeds & Northup, NorthWales, Pa., U.S.A.). The average particle size (APS) is defined as thevalue at which 50% of the particles are larger. The largest particlesize (LPS) is defined as the largest detectable particle on a Microtrachistogram.

Observed Density (ρobs) was calculated by dividing the observed weightin grams by the calculated volume in cubic centimeters (cc). The volumeof a sample was calculated by multiplying the average thickness, lengthand width. Each average comprised at least 5 separate measurements. Theuncertainty associated with these measurements was carried throughoutthe calculations.

Calculated Density (ρcalc) was calculated by the following equation:ρcalc=Σ(vi)*(ρi); where vi is the volume fraction of the i^(th)component, and ρi is the density of the i^(th) component.

Dielectric constant (Dk) at frequencies less than 3 GHz was obtainedusing a Hewlett-Packard 8753A Network Analyzer (Hewlett-Packard Corp.,San Jose, Calif.) by the substrate resonance method on a copper-cladlaminate.

Dielectric constant (Dk) and Dissipation Factor (Df) at frequenciesabove 5 GHz were obtained using a resonant mode dielectrometer developedby GDK products (GDK Products, Inc., Cazoniva, N.Y.) and a HewlettPackard 8510 Network Analyzer (Hewlett-Packard Crop., San Jose, Calif.).

Copper Peel values were determined using a 90-degree peel configurationon a copper-clad laminate anchored to a rigid sliding plane substratecoupled to an Applied Test Systems Model No. 1401 computer-controlledtensile testing machine (Applied Test Systems, Inc., Butler, Pa.,U.S.A.).

Compositions by weight were determined by thermalgravimetric analysis(TGA) using a TA High Resolution Thermalgravimetric Analyzer Model No.2950 linked to a TA Thermal Analyst CPU model No. 2000. (TA instruments,Wilmington, Del., U.S.A.). Each composite displayed stepwisedecomposition, each step being attributable to a separate component. Theuncertainty associated with this measurement was carried through anycalculations.

Void volume (VV) or "volume percent air" was calculated by dividing theobserved density by the calculated density and subtracting from unity,while propagating the appropriate degree of uncertainty.

Volume fraction (VF) of each component was calculated multiplying thevolume of massing the composite (1-VV) by the volume fraction of eachrespective component. It is calculated by the following equation: VF_(i)=(1-VV)*(Volume of i^(th) component/Total volume ofcomposite)=([(ρobs)/([ρcalc)]*[((W_(i))*(ρi)](VV+Σ(Wi)(ρi)]; whereVF_(i) is the volume fraction of the i^(th) component, ρobs is theobserved density in g/cc, ρcalc is the calculated density in g/cc, w_(i)is the weight fraction of the i^(th) component and ρi is the density ofthe i^(th) component in g/cc.

Voltage Breakdown was measured using a Model RM215-L/Z Breakdown Leakage& Ionization Tester (Biddie Instruments, Plymouth meeting, Pa.) byplacing a sheet of material between two parallel polished 4"×4" copperplates then gradually increasing DC potential between said plates untilelectrical breakdown was observed through the thickness of the material.Breakdown is defined as 1 milliamp current passing through the plates.

The present invention will now be described with reference to thefollowing non-limiting examples.

EXAMPLES Example 1

A slurry of 25.2 lb BaTiO3/SrTiO3 (Fuji Titanium N-5500; first treatedwith a 0.5% (w/w) of diphenylmethyl silane, Huls #P189) in 8 liters ofisopropanol was passed through a 325 mesh screen into a 40 litercontainer. While the slurry was agitated at about 300 rpm, 9.2 lb PTFEin the form of a 21.5% solids dispersion was rapidly poured into themixing vessel. The PTFE dispersion was an aqueous dispersion obtainedfrom Du Pont Company. The mixture was self-coagulating and within 5minutes co-coagulation was complete. The coagulum was gently poured overa porous cheesecloth and allowed to air dry. The flitrate from thisprocess was clear.

The coagulum was dried at 165° C. for 21 hours in a convection oven. Thematerial dried in small, cracked cakes approximately 2 cm thick and waschilled. The chilled cake was hand-ground using a tight, circular motionand minimal downward force through a 0.635 cm mesh stainless steelscreen, then 0.2 g of mineral spirits per gram of powder was added. Themixture was chilled, again passed through a 0.635 cm. mesh screen,tumbled for 5 minutes, then allowed to sit at 18° C. for 48 hours andwas re-tumbled for 5 minutes.

A 40 pound pellet was formed in a cylinder by pulling a vacuum andpressing at 860 psi. The pellet was then heated in a sealed tube at 49°C. for 16 hours. The pellet was then extruded into a 6"×0.080" tapeform. The tape was cut half-way along its length, and the two layersplied against one another. The resultant tape was calendered to athickness of 0.064", folded upon itself again to make a 4-ply tape,which was further calendered to a thickness of 0.025 inch. The lubricantwas evaporated by running the tape across heated rolls. The tape wasstretched in the machine direction twice: first at a 2 to 1 ratio, 250°C., 40 ft/min. The second stretch is also a 2 to 1 ratio, 250° C., 40ft/min. The expanded tape was then expanded transversely at an 10 to 1ratio, 270° C., 60 ft/min. to attain a 60"×0.0018" film. The film had nopinholes, an average weight of 33 g/m² porosity of 82%, and a Gurleyvalue of 14 sec.

The expanded filled film was then dipped into a 8.5% solids bath of amanganese-catalyzed bis-triazine resin (BT2060BJ, Mitsubishi GasChemical) in MEK and dried under tension for 1 min. at 160° C. Threeplies of this prepreg were laid up between copper foil and pressed at400 psi in a vacuum-assisted hydraulic press at temperature of 225° C.for 90 minutes then cooled under pressure. This resulting dielectricdisplayed good adhesion to copper, dielectric constant (@ 10 GHz) of12.7, dissipation factor of 0.005 (@ 10 GHz) and a capacitance of 1820picofarads/in² at an average thickness of 0.0015". The dielectric layerdisplayed a voltage breakdown strength of 800 volts/mil, and density of2.46 g/cm³.

Example 2

A slurry of 25.2 lb BaTiO3/SrTiO3 (Fuji Titanium N-5500; first treatedwith a 0.5% (w/w) of diphenylmethyl silane, Huls #P189) in 8 liters ofisopropanol was passed through a 325 mesh screen into a 40 litercontainer. While the slurry was agitated at about 300 rpm, 9.2 lb PTFEin the form of a 21.5% solids dispersion was rapidly poured into themixing vessel. The PTFE dispersion was an aqueous dispersion obtainedfrom Du Pont company. The mixture was self-coagulating and within 5minutes co-coagulation was complete. The coagulum was gently poured overa porous cheesecloth and allowed to air dry. The filtrate from thisprocess was clear.

The coagulum was dried at 165° C. for 21 hours in a convection oven. Thematerial dried in small, cracked cakes approximately 2 cm thick and waschilled. The chilled cake was hand-ground using a tight, circular motionand minimal downward force through a 0.635 cm mesh stainless steelscreen, then 0.2 g of mineral spirits per gram of powder was added. Themixture was chilled, again passed through a 0.635 cm mesh screen,tumbled for 5 minutes, then allowed to sit at 18° C. for 48 hours andwas re-tumbled for 5 minutes.

A 40 pound pellet was formed in a cylinder by pulling a vacuum andpressing at 860 psi. The pellet was then heated in a sealed tube at 49°C. for 16 hours. The pellet was then extruded into a 6"×0.080" tapeform. The tape was cut half-way along its length, and the two layersplied against one another. The resultant tape was calendered to athickness of 0.064", folded upon itself again to make a 4-ply tape,which was further calendered to a thickness of 0.025 inch. The lubricantwas evaporated by running the tape across heated rolls. The tape wasstretched in the machine direction twice: first at a 2 to 1 ratio, 250°C., 40 ft/min. The second stretch is also a 2 to 1 ratio, 250° C., 40ft/min. The expanded tape was then expanded transversely at an 10 to 1ratio, 270° C., 60 ft/min. to attain a 60"×0.0018" film. The film had nopinholes, an average weight of 33 g/m² porosity of 82%, and a Gudeyvalue of 14 sec.

The expanded filled film was then dipped into a 10% solids bath of adicyanamide/2-methylimidazole catalyzed flame retarded bisphenol-A basedpolyglycidyl ether (N-4002, Nelco Corp.) in MEK and dried under tensionfor 1 min. at 160° C. Three plies of this prepreg were laid up betweencopper foil and pressed at 250 psi in a vacuum-assisted hydraulic pressat temperature of 177° C. for 90 minutes then cooled under pressure.This resulted in a copper laminate having dielectric constant of 9.5,and a capacitance of 788 picofarads/in² at an average thickness of0.0027". The dielectric layer displayed a voltage breakdown strength of600 volts/mil.

Example 3

A slurry of 8,874 g of TiO2 (#203-1A, Transelco, a division of Ferro) in17.4 liters of water was stirred into a 40 liter container. While theslurry was agitated at about 300 rpm, 11,083 g PTFE in the form of a29.9% solids dispersion and 4450 g of a 0.4% solution of a cationicpolyacrylamide (Sedipur 802, BASF) was rapidly poured into the mixingvessel. The PTFE dispersion was an aqueous dispersion obtained from DuPont Company. The mixture was self-coagulating and within 5 minutesco-coagulation was complete. The coagulum was gently poured over aporous cheesecloth and allowed to air dry.

The coagulum was dried at 165° C. for 21 hours in a convection oven. Thematerial dried in small, cracked cakes approximately 2 cm thick and waschilled. The chilled cake was hand-ground using a tight, circular motionand minimal downward force through a 0.635 cm mesh stainless steelscreen, then 0.22 g of mineral spirits per gram of powder was added. Themixture was chilled, again passed through a 0.635 cm. mesh screen,tumbled for 5 minutes, then allowed to sit at 18° C. for 48 hours andwas re-tumbled for 5 minutes.

A 40 pound pellet was formed in a cylinder by pulling a vacuum andpressing at 840 psi. The pellet was then heated in a sealed tube at 49°C. for 16 hours. The pellet was then extruded into a 6"×0.030" tapeform. The tape was calendered to a thickness of 0.0059". The lubricantwas evaporated by running the tape across heated rolls. The tape wasstretched in the machine direction twice: first at a 1.5 to 1 ratio,275° C., 5 ft/min. The second stretch is also a 1.5 to 1 ratio, 275° C.,5 ft/min. The expanded tape was then expanded transversely at an 3 to 1ratio, 300° C., 20 ft/min to attain a 16"×0.003" film. The film had nopinholes, and average mean flow pore size of 19 micrometers.

A fine dispersion was prepared by mixing 15.44 kg TiO2 powder (TIPureR-900, DuPont Company) into a catalyzed solution of 3.30 kg bis triazineresin (BT206OBH, Mitsubishi Gas Chemical) and 15.38 kg MEK. Thedispersion was constantly agitated so as to insure uniformity. A swatchof 0.004" TiO2-filled expanded PTFE described above was then dipped intothe resin mixture, removed, and then dried at 165° C. for 1 min. undertension to afford a flexible composite. Several plies of this prepregwere laid up between copper foil and pressed at 500 psi in avacuum-assisted hydraulic press at temperature of 220° C. for 90 minutesthen cooled under pressure. This resulting dielectric displayed goodadhesion to copper, dielectric constant (@ 10 GHz) of 10.0 anddissipation factor (@ 10 GHz) of 0.008.

Example 4

A fine dispersion was prepared by mixing 83.5 g TiO2 powder (TIPureR-900, DuPont Company) into a catalyzed solution of 22.5 g of adicyanamide/2-methylimadazole catalyzed flame retarded bisphenol-A basedpolyglycidyl ether (N-4002, Nelco Corp.) in 90.2 g MEK. The dispersionwas constantly agitated so as to insure uniformity. A swatch of 5.0micrometer thick expanded PTFE was then dipped into the resin mixture,removed, and then dried at 165° C. for 1 min. under tension to afford aflexible composite. A single ply of this prepreg were laid up betweencopper foil and pressed at 34 psi in a vacuum-assisted hydraulic pressat temperature of 149° C. for 90 minutes then cooled under pressure.This resulting dielectric displayed good adhesion to copper, dielectricconstant (@ 1 MHz) of 8.7 and capacitance of 9,744 picofarads/in².

We claim:
 1. A high dielectric adhesive composite comprising:at leastone porous polytetrafluoroethylene substrate layer having a structure ofpolymeric nodes and fibrils and voids therebetween; a predeterminedamount of dielectric filler located within the composite to produce acomposite with a dielectric constant of at least 4.5; and an adhesiveimbibed within the voids of said porous expanded substrate layer, theadhesive comprising less than 50 volume percentage of the composite. 2.A composite according to claim 1, wherein the composite has acapacitance of at least 700 picofarads/in² and a voltage breakdown ofgreater than 500 volts/mil.
 3. A composite according to claim 1, whereinsaid substrate layer is expanded polytetrafluoroethylene.
 4. A compositeaccording to claim 1, wherein said substrate layer is an expandedpolytetrafluoroethylene containing a tetrafluoroethylene copolymer.
 5. Acomposite according to claim 1, wherein said high dielectric filler isselected from the group consisting of calcium titanate, barium titanate,strontium titanate, or a mixture of said titanates.
 6. A compositeaccording to claim 1, wherein said adhesive is a thermoset orthermoplastic resin.
 7. A composite according to claim 1, wherein saidadhesive is selected from the group consisting of epoxy resin, cyanateester resin, or polybutadiene resin.
 8. A laminate comprising a metalliclayer adhered to the composite according to claim
 1. 9. A laminateaccording to claim 8, wherein said metallic layer is a layer of copperfoil.
 10. The composite according to claim 1, wherein the substratecomprises about 10 to 40 volume percent of said composite.
 11. Thecomposite according to claim 1, wherein the dieletric filler comprisesabout 15 to 55 volume percent of the composite.
 12. The compositeaccording to claim 1, wherein the resin comprises about 15 to 40 volumepercent of the composite.
 13. A capacitor comprisinga metal foil layerlaminated to at least one side of a composite formed from at least oneporous fluoropolymer substrate layer having a node-fibril structure;dielectric filler located within at least the nodes; and an adhesiveimbibed within within the fluoropolymer substrate within voids betweenthe nodes and fibrils, the adhesive comprising less than 50 volumepercentage of the composite.